Final answer:
An object will achieve neutral buoyancy when its average density equals the fluid density. The concept of specific gravity, which is less than one for floating objects and more than one for sinking objects, is central to determining an object's buoyant state in a fluid.
Step-by-step explanation:
An object will be in neutral buoyancy at any submerged depth in a fluid if the average density of the object and the density of the fluid are equal. When the object has a density less than the fluid, it will float, because the buoyant force, which is equal to the weight of the displaced fluid, is greater than the weight of the object. Conversely, an object denser than the fluid will sink. The exact fraction submerged is the ratio of the volume submerged to the volume of the object, also known as the object's specific gravity.
Specific gravity, which is the average density of the object (Pobj) divided by the density of the water (pfl), is a crucial concept in understanding buoyancy. If an object's specific gravity is less than one, it floats; if more than one, it sinks. If the specific gravity is exactly one, the object will be neutrally buoyant, neither sinking nor floating, which is a condition scuba divers aim for to hover in the water comfortably. Specific gravity can also be used to assess the condition of various fluids, such as battery acid or radiator fluid.